Field of the Invention
The present invention relates to methods and apparatus for metrology usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, a measure of the accuracy of alignment of two layers in a device. Overlay may be described in terms of the degree of misalignment between the two layers, for example reference to a measured overlay of 1 nm may describe a situation where two layers are misaligned by 1 nm.
Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target structure and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
The target structures used by conventional scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). This simplifies mathematical reconstruction of the target structure as it can be regarded as infinite. However, in order to reduce the size of the target structures, e.g., to 10 μm by 10 μm or less, e.g., so they can be positioned in amongst product features, rather than in the scribe lane, metrology has been proposed in which the grating is made smaller than the measurement spot (i.e., the grating is overfilled). Typically such target structures are measured using dark field scatterometry in which the zeroth order of diffraction (corresponding to a specular reflection) is blocked, and only higher orders processed. Examples of dark field metrology can be found in international patent applications WO 2009/078708 and WO 2009/106279 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in patent publications US20110027704A, US20110043791A and US20120242970A. The contents of all these applications are also incorporated herein by reference. Diffraction-based overlay using dark-field detection of the diffraction orders enables overlay measurements on smaller target structures. These target structures can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Target structures can comprise multiple targets which can be measured in one image.
In the known metrology technique, overlay measurement results are obtained by measuring the target structure twice under certain conditions, while either rotating the target structure or changing the illumination mode or imaging mode to obtain separately the −1st and the +1st diffraction order intensities. The intensity asymmetry, a comparison of these diffraction order intensities, for a given target structure provides a measurement of target asymmetry, that is asymmetry in the target structure. This asymmetry in the target structure can be used as an indicator of overlay error (undesired misalignment of two layers).
Although the known dark-field image-based overlay measurements are fast and computationally very simple (once calibrated), they rely on an assumption that overlay (i.e., overlay error and deliberate bias) is the only cause of target asymmetry in the target structure. Any other asymmetry in the target structure, such as structural asymmetry of features within one or both of the overlaid gratings, also causes an intensity asymmetry in the 1st (or other higher) orders. This intensity asymmetry attributable to structural asymmetry, and which is not related to overlay, clearly perturbs the overlay measurement, giving an inaccurate overlay measurement. Asymmetry in the lowermost or bottom grating of a target structure is a common form of structural asymmetry. It may originate for example in wafer processing steps such as chemical-mechanical polishing (CMP), performed after the bottom grating was originally formed.
Therefore, it is desired to distinguish the contributions to target asymmetry that are caused by overlay error and other effects in a more direct and simple way.